1. Field of the Invention
The present invention relates to a power supply apparatus.
2. Description of the Related Art
A small-sized information terminal such as a recent cellular telephone or a PDA (personal digital assistance) includes a device, for example, an LED (light emitting diode) employed as a backlight of a liquid crystal, that needs higher voltage than a voltage output from a battery. For example, a lithium-ion (Li-ion) battery is often employed in a small-sized information terminal of this type. A voltage output is generally about 3.5 volts (V) and the voltage output from the Li-ion battery even when the Li-ion battery is fully charged is as low as about 4.2 V. The LED needs a voltage higher than the battery voltage as a driving voltage. In this way, if the higher voltage than the battery voltage is required, the voltage necessary to drive a load circuit such as the LED is obtained by boosting the battery voltage using a switching regulator or a booster circuit such as a charge pump circuit (see Japanese Patent Application Laid-Open No. 2001-223095).
Meanwhile, a control circuit controls a boosting operation performed by such a booster circuit by feeding back a voltage at a certain terminal on the load circuit to the control circuit so that the load circuit can stably operate. For example, to drive the LED, the following method may be considered. A constant current source for driving the LED is connected to a cathode terminal of the LED, and the control circuit controls the cathode terminal so that a voltage at the cathode terminal is kept to a constant value (=Vx) while maintaining the brightness of the LED. The voltage Vx is decided so as not to saturate the constant current source. At this time, the voltage at the cathode terminal is fed back to the control circuit of the booster circuit, whereby the control circuit controls the booster circuit so that the voltage at the cathode terminal coincides with the voltage Vx.
Under these circumstances, the inventor of the present invention has recognized the following problems. According to the conventional method, a voltage at an anode terminal of the LED, that is, an output voltage of the booster circuit is stabilized to Vf+Vx while the constant current circuit applies current to the LED, where Vf is a forward voltage flowing through the LED.
When the LED is switched to an off state, the output voltage gradually lowers if a discharge path from an output capacitor of the booster circuit is present besides the LED. If no other discharge path than the LED is present, then electric charges are held in the output capacitor, and the output voltage is kept almost constant. Namely, the output voltage is according to a load state, that is, variable. If the output voltage is variable, the variable output voltage may possibly influence functions of the other circuits such as short-circuit detection and protection circuits. As a result, the booster circuit disadvantageously turns unstable as a whole.